1. Field of the Invention
The present invention relates to a conjugate of
i) a biologically active peptide or protein which can be bound to a specific receptor of a cell in a human body, and PA1 ii) a high-molecular weight substance having an affinity for a nucleic acid, especially a DNA, such that the nucleic acid can be adsorbed or dispersed into the same, as well as a process for producing said conjugate. PA1 i) The nucleic acid should be delivered to the cell such that it is not easily decomposed. PA1 ii) The nucleic acid should be transferred into the cell at a high rate and, as much as possible, to the specific cell targeted for therapy.
The composite can bind to a cell having a complementary receptor and can transfer a nucleic acid into a specific type of cells, a specific organ or tumor cells of the human body.
2. Discussion of the Background
It has been considered that biologically active peptides can be used as target molecules for drug delivery. For example, a method has been described in which diphtheria toxin was bound to an IL-6 molecule and specifically sent to tumor cells having an IL-6 receptor to thereby kill the tumor cells. Further, a method was described in which an anti-tumor agent was bound to a monoclonal antibody that recognized a cancer-specific antigen and was specifically sent to tumor cells.
Methods using biologically active peptides in drug delivery systems (DDS) have become important in the field of gene therapy. Wu et al have developed a method for gene therapy in which polylysine (having a positive charge) is chemically bound to a biologically active peptide such as asialoorosomucoid to form a conjugate. A DNA plasmid (having a negative charge) can then be adsorbed into this conjugate to form a composite. Such a conjugate can be used to send the DNA to a hepatocyte having an asialoglycoprotein receptor (J. Biol. Chem., 236, 14621, (1988)). This conjugate can transfer the gene into the liver following intravenous injection and is expected to be an efficient gene delivery system.
The diseases which can be treated with such specific composites depend on the gene which is delivered. For example, a viral thymidine kinase gene targeted into the liver can activate the anti-viral drug, ganciclovir, directly in the liver. Such directed therapy results in less side effects.
When a nucleic acid is administered to the human body for gene therapy two difficulties arise. First, the nucleic acid is rapidly decomposed. Second, the DNA cannot easily enter a cell. Further, when the nucleic acid is inserted into the genome, the cell can be adversely affected by insertion of the DNA at a site which impairs critical gene expression or by expression of a product which impairs the functioning of the cell. Accordingly, the following elements are desirable in a composition suitable for delivering nucleic acids:
As one solution to the above-mentioned problems, the nucleic acid has been adsorbed into a polymer which suppresses the decomposition of the nucleic acid. As another solution, a ligand which binds to a specific receptor on the targeted cell is combined with a polymer with the nucleic acid adsorbed therein. In both of these, the composite comprising the nucleic acid adsorbed into the polymer is transported to the target cell type by means of endocytosis through ligand-receptor binding. In this case, it is necessary to promote binding of the ligand, which is the composite comprising the nucleic acid adsorbed in the polymer, to the biologically receptor.
Unfortunately, when the biologically active peptide is chemically bound to the polymer, its biological activity tends to be impaired and by-products are formed through various side reactions. This reduces the quality of the drug system. It is desirable to develop a practical method for binding a high-molecular weight substance having a nucleic acid adsorbed therein to a biologically active peptide specifically under mild conditions. It is also desirable to find such a process which has few side reactions.